1. Field of the Invention
The present invention relates to a turn control system for a four wheel drive vehicle, particularly an earth working vehicle such as a tractor.
2. Description of the Related Art
To promote operating efficiency of the earth working vehicle in repeating U-turns during an operation, it is important to minimize a space required when the vehicle changes its running directions. Japanese Patent Publication Kokai No. 1988-34229 and U.S. Pat. No. 4,723,622 disclose four wheel drive vehicles capable of making such small, sharp turns. These vehicles comprise;
an engine, PA1 right and left non-steerable wheels for receiving drive of the engine through a differential device, PA1 right and left steerable wheels for receiving the drive of the engine through a further differential device, PA1 a steering device for controlling the steerable wheels, PA1 a change speed device connected to the steerable wheels, the change speed device including a standard transmission mode for driving the right and left steerable wheels at substantially the same average peripheral velocity as the right and left non-steerable wheels, an accelerating transmission mode for driving the right and left steerable wheels at a greater average peripheral velocity than the right and left non-steerable wheels, and switching device for selecting between the standard transmission mode and the accelerating transmission mode, and PA1 an interlocking device for interlocking the steering device and the switching device such that the change speed device is operable in the accelerating transmission mode when the steerable wheels are steered in excess of a predetermined angle.
This type of four wheel drive vehicle in practical use today employs a steering specification for the four wheel drive tractor which has been widely used in the past. As described in Japanese Patent Publication Kokai No. 1988-34229 mentioned above and in U.S. Pat. No. 4,696,365, its maximum steering angle is set to about 55 degrees. With such a steering specification, a side force is generated, when the vehicle makes a turn, to push front wheels, which are the steerable wheels, outwardly with respect to the turning circle since the front wheels are accelerated then. This results in an increased resistance to running of the vehicle, or leaves marred patches of ground.
Further, as is known from Japanese Patent Publication Kokai No. 1988-287630 and U.S. Pat. No. 4,669,559, an inner one of the rear wheels with respect to a turning circle is locked by a side brake besides accelerating the front wheels in order to make the turning circle smaller. This is a forced turn, though a small turn it may be, which is injurious to the ground and causes a considerable loss of power.